1. Technical Field
The present disclosure relates to a light pulse generator and an optical time domain reflectometer using the light pulse generator. More specifically, the present disclosure relates to a light pulse generator which outputs light pulses having a steep rising edge and an optical time domain reflectometer using the light pulse generator.
2. Related Art
In optical communication systems which perform a data communication using an optical signal, it is important to monitor optical fibers for transmitting optical signals. An optical time domain reflectometer (hereinafter abbreviated as “OTDR”) is used in installation, maintenance or the like of optical fibers.
The OTDR performs measurements relating to a disconnection, a loss or the like of an optical fiber to be measured by providing repetitive light pulses to the optical fiber from a measurement connector provided at the entrance/exit end of the OTDR and measuring levels and reception times of return light beams (reflection light beams, back scattering light beams, etc.) coming from the optical fiber.
A light pulse generator is used in OTDRs as a light source for emitting light pulses to an optical fiber to be measured (see JP-A-2008-089336 and JP-A-2008-107319, for example).
FIG. 9 is a circuit diagram showing the configuration of a related-art light pulse generator. As shown in FIG. 9, the light pulse generator includes: a laser diode 11; a transistor 12; a constant current source 13; a constant voltage source 14; and a modulation control signal source 15. The light pulse generator emits light pulses. The laser diode 11, the transistor 12, the constant current source 13, and the constant voltage source 14 form a closed loop.
The laser diode (LD) 11 emits light pulses for measurement of an optical fiber.
The transistor 12, which is a switching element, is turned on/off (i.e., conduction between its collector and emitter is established/canceled) in response to a control signal that is provided to the base from the modulation control signal source 15. The collector of the transistor 12 is connected to the cathode of the LD 11.
The constant current source 13, one end of which is connected to the emitter of the transistor 12, causes a flow of a constant emitter current while the transistor 12 is on ((emitter current)≅(collector current)).
The constant voltage source 14, the positive pole side of which is connected to the anode of the LD 11, forward-biases the LD 11.
The modulation control signal source 15 provides, to the base of the transistor 12, a modulation control signal for turning on/off the transistor 12.
The closed loop formed by the LD 11, the transistor 12, the constant current source 13, and the constant voltage source 14 will be hereinafter referred to as a forward current loop. A current that flows through the LD 11 in the forward direction in the forward current loop will be hereinafter referred to as an LD forward current Id.
The components 11-14 are mounted on a printed circuit board(s) or the like and electrically connected to each other by printed interconnections on the board, cables connecting the boards, etc. As a result, inductances L1-L4 occur in the interconnections and cables connecting the components 11-14. In other words, the series wiring inductances L1-L4 exist in the forward current loop.
The operation of the above laser pulse generator will be described below.
The modulation control signal source 15 provides, to the base of the transistor 12, a modulation control signal for turning on/off the transistor 12. The transistor 12 is turned on when the level of the modulation control signal is changed from low to high, and the transistor 12 is kept on while the modulation control signal is kept at the high level.
While the transistor 12 is on, the LD 11 is forward-biased by the constant voltage source 14 and the LD forward current Id (the constant current of the constant current source 13) flows through the LD 11. The LD 11 emits laser light if the LD forward current Id is larger than its threshold current.
On the other hand, the transistor 12 is turned off when the level of the modulation control signal supplied from the modulation control signal source 15 is changed from high to low, and the transistor 12 is kept off and the forward current loop is kept open while the modulation control signal is kept at the low level. In this state, the LD forward current Id is shut off and the LD 11 does not emit laser light.
As described above, the LD forward current Id of the LD 11 is caused to flow or shut off by tuning on or off the transistor 12. The LD 11 is caused to emit light pulses by directly intensity-modulating in the LD 11.
FIG. 10 is a graph showing a laser light emission characteristic of the LD 11, wherein the horizontal axis represents the LD forward current Id and the vertical axis represents the laser output optical power. The laser output power increases as the LD forward current Id increases after it exceeds the threshold current.
To cause the LD 11 to emit a light pulse, it is necessary to cause a pulse-shaped LD forward current that is proportional to an optical power of laser light to flow through the LD 11. It is possible, by using electronic components on the market, to cause the modulation control signal source 15 to generate a control signal having a pulse width of several nanoseconds and provide it to the transistor 12 and to cause the transistor 12 to be turned on/off in response to such a modulation control signal.
On the other hand, the printed circuit board on which the components 11-14 are mounted and the components 11-14 themselves have the inductances L1-L4 as shown in FIG. 9.
An inductance L′ in the forward current loop and the LD forward current Id which flows through the forward current loop while the transistor 12 is on are given by the following Equations (1) and (2), respectively.L′=L1+L2+L3+L4  (1)Id=Ton·(E1−Vf)/L′  (2)
In Equations (1) and (2), L′ is a combined wiring inductance of the inductances L1-L4. Ton is the elapsed time from turn-on of the transistor 12, E1 is the voltage of the constant voltage source 14, and Vf is the voltage between the two terminals of the LD 11 (i.e., the forward voltage of the LD 11).
Therefore, ΔId/ΔTon is restricted by the forward current loop during a period from turn-on of the transistor 12 to a start of laser light emission of the LD 11. That is, it is difficult to generate a laser light pulse having a steep rising edge using a large current.
Further, it is difficult to remove the inductances of the components 11-15 and the printed circuit board on which they are mounted, and hence shortening of the width of a light pulse becomes more difficult as the necessary LD forward current Id increases (i.e., as the necessary output optical power of the LD 11 increases).